There is a considerable need in both the military and commercial sectors for quiet, efficient and lightweight power sources that have improved power density. Military applications include, but are not limited to, submersibles, surface ships, portable/mobile field generating units, and low power units (i.e., battery replacements). For example, the military has a strong interest in developing low range power sources (a few watts to a few kilowatts) that can function as replacements for batteries. Commercial applications include transportation (i.e., automotive, bus, truck and railway), communications, on-site cogeneration and stationary power generation.
Other interest exists for household applications, such as radios, camcorders and laptop computers. Additional interest exists in larger power sources or sources of higher power density that can be used in operating clean, efficient vehicles. In general, there is a need for quiet, efficient and lightweight power sources anywhere stationary power generation is needed.
Additionally, the use of gasoline-powered internal combustion engines has created several environmental, exhaust gas-related problems. One possible solution to these environmental problems is the use of fuel cells. Fuel cells are highly efficient electrochemical energy conversion devices that directly convert the chemical energy derived from renewable fuel into electrical energy.
Significant research and development activity has focused on the development of proton-exchange membrane fuel cells. Proton-exchange membrane fuel cells have a polymer electrolyte membrane disposed between a positive electrode (cathode) and a negative electrode (anode). The polymer electrolyte membrane is composed of an ion-exchange polymer (i.e., ionomer). Its role is to provide a means for ionic transport and prevent mixing of the molecular forms of the fuel and the oxidant.
Solid polymer electrolyte fuel cells (SPEFCs) are an ideal source of quiet, efficient, and lightweight power. While batteries have reactants contained within their structure which eventually are used up, fuel cells use air and hydrogen to operate continuously. Their fuel efficiency is high (45 to 50 percent), they do not produce noise, operate over a wide power range (10 watts to several hundred kilowatts), and are relatively simple to design, manufacture and operate. Further, SPEFCs currently have the highest power density of all fuel cell types. In addition, SPEFCs do not produce any environmentally hazardous emissions such as NOx and SOx (typical combustion by-products).
The traditional SPEFC contains a solid polymer ion-exchange membrane that lies between two gas diffusion electrodes, an anode and a cathode, each commonly containing a metal catalyst supported by an electrically conductive material. The gas diffusion electrodes are exposed to the respective reactant gases, the reductant gas and the oxidant gas. An electrochemical reaction occurs at each of the two junctions (three phase boundaries) where one of the electrodes, electrolyte polymer membrane and reactant gas interface.
Despite their potential for many applications, SPEFCs have not yet been commercialized due to unresolved technical problems and high overall cost. One major deficiency impacting the commercialization of the SPEFC is the inherent limitations of today's leading membrane and electrode assemblies. To make the SPEFC commercially viable (especially in automotive applications), the membranes employed must operate at elevated/high temperatures (>120° C.) so as to provide increased power density, and limit catalyst sensitivity to fuel impurities. This would also allow for applications such as on-site cogeneration (high quality waste heat in addition to electrical power). Current membranes also allow excessive methanol crossover in liquid feed direct methanol fuel cells (dependent on actual operating conditions, but is typically equivalent to a current density loss of about 50 to 200 mA/cm2 @ 0.5V). This crossover results in poor fuel efficiency as well as limited performance levels.
Several polymer electrolyte membranes have been developed over the years for application as solid polymer electrolytes in fuel cells. However, these membranes have significant limitations when applied to liquid-feed direct methanol fuel cells and to hydrogen fuel cells. The membranes in today's most advanced SPEFCs do not possess the required combination of ionic conductivity, mechanical strength, dehydration resistance, chemical stability and fuel impermeability (e.g., methanol crossover) to operate at elevated temperatures.
DuPont developed a series of perfluorinated sulfonic acid membranes known as Nafion® membranes. The Nafion® membrane technology is well known in the art and is described in U.S. Pat. Nos. 3,282,875 and 4,330,654. Unreinforced Nafion® membranes are used almost exclusively as the ion exchange membrane in present SPEFC applications. This membrane is fabricated from a copolymer of tetrafluoroethylene (TFE) and a perfluorovinyl ethersulfonyl fluoride. The vinyl ether comonomer is copolymerized with TFE to form a melt-processable polymer. Once in the desired shape, the sulfonyl fluoride group is hydrolyzed into the ionic sulfonate form.
There are several mechanisms that limit the performance of Nafion® membranes in fuel cell environments at temperatures above 100° C. In fact, these phenomenon may begin at temperatures above even 80° C. Mechanisms include membrane dehydration, reduction of ionic conductivity, radical formation in the membrane (which can destroy the solid polymer electrolyte membrane chemically), loss of mechanical strength via softening, and increased parasitic losses through high fuel permeation.
The Nafion® membrane/electrode is also very expensive to produce, and as a result it is not (yet) commercially viable. Reducing membrane cost is crucial to the commercialization of SPEFCs. It is estimated that membrane cost must be reduced by at least an order of magnitude from the Nafion® model for SPEFCs to become commercially attractive.
In an effort to reduce costs and move toward potential commercialization of SPEFCs, ion-exchange membranes that are less expensive to produce also have been investigated for use in polymer electrolyte membrane fuel cells.
Sulfonated poly(aryl ether ketones) developed by Hoechst AG are described in European Patent No. 574,891,A2. These polymers can be crosslinked by primary and secondary amines. However, when used as membranes and tested in polymer electrolyte membrane fuel cells, only modest cell performance is observed.
Sulfonated polyaromatic based systems, such as those described in U.S. Pat. Nos. 3,528,858 and 3,226,361, also have been investigated as membrane materials for SPEFCs. However, these materials suffer from poor chemical resistance and mechanical properties that limit their use in SPEFC applications.
Solid polymer membranes comprising a sulfonated poly(2,6 dimethyl 1,4 phenylene oxide) alone or blended with poly(vinylidene fluoride) also have been investigated. These membranes are disclosed in WO 97/24777. However, these membranes are known to be especially vulnerable to degradation from peroxide radicals.
The inherent problems and limitations of using solid polymer electrolyte membranes in electrochemical applications, such as fuel cells, at elevated/high temperatures (>100° C.) have not been solved by the polymer electrolyte membranes known in the art. Specifically, maintaining high ion conductivity and high mechanical strength, resisting dehydration and other forms of degradation remain problematic, especially at elevated operating temperatures. As a result, commercialization of SPEFCs has not been realized.
Composite polymer membranes having an ion conducting material dispersed in a microporous substrate polymer have been the subject of substantial research. Several membranes are disclosed in, U.S. Pat. No. 6,248,469 in which an ion conducting membrane is incorporated into pores of a microporous membrane composed of a rigid rod or extended rod lyotropic liquid crystalline polymer. Microporous membranes were traditionally prepared from as-polymerized dope solutions in part because the no methods had previously been disclosed to prepare lower concentration dope solutions of high molecular weight lyotropic liquid crystalline polymers by either lower concentration polymerization or by dilution of a high concentration dope.
Lyotropic liquid crystal polymers such as polyphenylene benzobisoxazole (PBO) and polyphenylene benzobisthiazole (PBZT) can be formed into fibers that exhibit the highest tensile strength of any polymeric fiber (>500 ksi) These so-called “rigid rod” polymers when spun into fibers, coagulated and dried also exhibit excellent high temperature capability (up to 550° C.) and excellent resistance to organic solvents and most common acids. The spin dopes of these materials are supplied as high molecular weight (intrinsic viscosity >24 dL/g) nematic phase solutions of the polymer in polyphosphoric acid (PPA). The typical spin dope contains 12-15% polymer by weight.
High strength biaxially oriented thin films of these materials have been recited in U.S. Pat. Nos. 4,963,428, 4,973,442, and 6,132,668 issued to Foster-Miller, Inc., which films are prepared using a modified blown film extrusion process using a novel counter-rotating circular die to impart shear orientation to the nematic phase polymer. It has also been demonstrated that these biaxially oriented films when coagulated in water or some other non-solvent can form extremely strong open membranes when preserved in the wet, swollen state. Using solvent exchange techniques it has been shown that a number of different materials such as sol gel glasses, polymers, including ion conducting polymers, and other materials etc. can be infused or imbibed into this membrane structure. When dried, the liquid crystal polymer network shrinks (consolidates) around the imbibed material forming a very high strength structure that preserves the functional properties of the imbibed material.
U.S. Pat. No. 6,248,469 recites composite membranes and various methods of making same wherein ion conducting polymers (ICPs) are incorporated into the microporous PBO structure to produce a microcomposite proton exchange membrane (PEM) that would overcome the cost/performance problems of perfluorinated PEMs based on Dupont's Nafion polymer. Nafion type solid polymer electrolyte membranes exhibit long term stability of >20,000 hrs but cannot meet the needs for an automotive fuel cell due to high cost (˜$770/m2) and serious degradation of mechanical strength at operating temperatures of greater than 100° C. The '469 patent recited various composite membranes having a relatively low cost sulfonated aromatic polymers, such as sulfonated polyether sulfones or sulfide/sulfones, imbibed into an appropriate PBO membrane support structure. These lower cost aromatic ion conductors address the PEM cost problem and the PBO support structure addresses the mechanical strength issues, permitting fuel cell operation at optimally efficient temperatures between 100 to 150° C. These higher operating temperatures improve fuel cell efficiency, increase specific power and reduce the negative effect of CO on membrane electrode (MEA) platinum catalyst activity. Although composite membranes prepared from commercially available spin dope of 14.6 wt % PBO in PPA can produce a membrane support film with outstanding mechanical properties and increased electrochemical performance, still greater electrochemical output is still desirable.
Initial theoretical calculations indicated that a dope containing approximately 6-8 wt % PBO in PPA would yield membranes with sufficient open pore volume while retaining sufficient mechanical strength to produce composite membranes suitable for use in high electrochemical density fuel cell applications. Unfortunately, attempts to dilute the extremely viscous 14.6% PBO/PPA dope with additional PPA were not successful. Heterogeneous mixtures of PBO in PPA were produced which were not suitable for quality film preparation. Similarly, attempts to prepare 8% dope solutions of PBO in PPA by polymerization at a reduced reaction concentration resulted in mixtures which were not suitable for film/membrane production.
Very low concentration (˜1 wt %) PBO solutions in methane sulfonic acid (MSA) have been prepared. MSA is a very strong acid and best known solvent for the lyotropic rigid rod polymers. PBO fiber can be dissolved in MSA using laboratory glassware and mixing equipment. This yielded a relatively low viscosity isotropic phase solution which could be used to make hand cast membranes with open pore volumes, e.g., membranes with greater than 90% porosity. Composite membranes comprising a variety of ion conducting polymers imbibed into PBO membranes prepared from the 1% in MSA dope solution have been prepared and exhibit electrochemical activity similar to Nafion membranes in single cell fuel cell test conducted at 80° C. Attempts to use composite membranes prepared from the 1% dope solution in fuel cells at temperatures greater than 80° C. were unsuccessful in part because the low concentration isotropic phase solutions in MSA did not have sufficient mechanical strength. Higher concentration PBO in MSA solutions cannot be obtained due to solubility and miscibility limitations of PBO above 1% by weight in MSA.
In view of these and other limitations observed in the art, it would be highly desirable to develop rigid rod or extended rod lyotropic liquid crystalline polymer dope solutions having a concentration less than that of the as-polymerized dope and greater than 1%. It would also be desirable to provide methods of making diluted dope solutions from high concentration as-polymerized solutions. It would also be desirable to develop improved solid polymer electrolyte membrane with high resistance to dehydration, high mechanical strength and stability to a range of operating temperatures of at least between at 80° C. and 150° C., more preferably between about 50° C. and 200° C. using diluted rigid rod or extended rod lyotropic liquid crystalline polymer dope solutions.